1. Field of the Invention
This invention relates to valves passively actuated by shape memory alloy elements in response to temperature conditions, and particularly to such valves which respond to ambient temperature conditions rather than to the temperature of the fluid passing through the valves.
2. Background Information
Shape memory alloys possess interesting and potentially useful properties. These alloys undergo a crystalline phase change known as thermoelastic martensitic transformation. Below the transformation temperature these alloys are martensitic and can be rather easily deformed. When heated above the transformation temperature the original shape is restored and the material is converted to its high strength, austenitic condition. The transformation occurs rapidly at the critical temperature and develops considerable stress.
Shape memory alloys have been utilized in various actuating devices. For instance, they have been used to make springs responsive to fluid temperature to operate valves controlling flow of the fluid. They have also been used as beams which bend to actuate switches at certain temperatures, and to operate vents in greenhouses in response to temperature. Among other uses of shape memory alloys are temperature operated tweezers, flappers for fluid valves, and valves in automatic transmissions.
There are applications, such as for instance a nuclear reactor installation, for passively temperature activated valves which respond to ambient temperature conditions, rather than the temperature of the fluid controlled by the valve. These valves can be quite large and require a commensurate stroke. In some applications, the valves must be positively retained in a fully closed or fully opened position under normal conditions for reliability and safety. Often this is achieved by the use of large biasing springs which increase the force required to operate such valves. In many instances, the valve must be positively maintained in the operated condition.
There is, therefore, a need for a passively temperature actuated valve responsive to ambient temperature conditions.
This includes the need for such a passively ambient temperature actuated valve which can develop the force and stroke needed to operate sizable valves such as those used in nuclear reactor systems.
There is also a need for such a valve which once actuated in response to a rise in temperature above a predetermined temperature, remains actuated even though the temperature later falls below the predetermined temperature.